At the present time, a wide variety of prints are prepared and used. The number of types of these prints tends to be increased. This is because the development and sophistication of society have led to a significantly broadened range of applications where prints are used, as well as to stricter demands to be satisfied by the prints. For example, the function of preventing duplication and the function of identifying the print as a genuine print are strictly required of prints, which should not be forged, such as ID cards or passports. Further, for commercial prints such as calendars and posters, in order to draw attention, a technique is sometimes used which enables an image to be formed only when the prints have been exposed to specific conditions.
To meet these demands, prints have been developed in which, upon exposure to specific light other than visible light, an image perceivable with the naked eye appears, although the image cannot be perceived with the naked eye under ordinary visible light. In these prints, a fluorescent ink, which emits visible light upon exposure to excitation light such as ultraviolet light, has been printed.
In recent years, however, the fluorescent ink and, in addition, black light as a portable ultraviolet light exposure system have become relatively easily available, and, consequently, prints utilizing the conventional fluorescent ink have become easy to forge or alter. This has disadvantageously made it difficult to guarantee a high level of security.